Re: [PATCH v7 22/25] ACPI / APEI: Kick the memory_failure() queue for synchronous errors

Borislav Petkov
2019-01-21T17:59:02Z

On Mon, Dec 03, 2018 at 06:06:10PM +0000, James Morse wrote:
> memory_failure() offlines or repairs pages of memory that have been
> discovered to be corrupt. These may be detected by an external
> component, (e.g. the memory controller), and notified via an IRQ.
> In this case the work is queued as not all of memory_failure()s work
> can happen in IRQ context.
>
> If the error was detected as a result of user-space accessing a
> corrupt memory location the CPU may take an abort instead. On arm64
> this is a 'synchronous external abort', and on a firmware first
> system it is replayed using NOTIFY_SEA.
>
> This notification has NMI like properties, (it can interrupt
> IRQ-masked code), so the memory_failure() work is queued. If we
> return to user-space before the queued memory_failure() work is
> processed, we will take the fault again. This loop may cause platform
> firmware to exceed some threshold and reboot when Linux could have
> recovered from this error.
>
> If a ghes notification type indicates that it may be triggered again
> when we return to user-space, use the task-work and notify-resume
> hooks to kick the relevant memory_failure() queue before returning
> to user-space.
>
> Signed-off-by: James Morse <james.morse@arm.com>
>
> ---
> current->mm == &init_mm ? I couldn't find a helper for this.
> The intent is not to set TIF flags on kernel threads. What happens
> if a kernel-thread takes on of these? Its just one of the many
> not-handled-very-well cases we have already, as memory_failure()
> puts it: "try to be lucky".
>
> I assume that if NOTIFY_NMI is coming from SMM it must suffer from
> this problem too.

Good question.

I'm guessing all those things should be queued on a normal struct
work_struct queue, no?

Now, memory_failure_queue() does that and can run from IRQ context so
you need only an irq_work which can queue from NMI context. We do it
this way in the MCA code:

We queue in an irq_work in NMI context and work through the items in
process context.

> ---
> drivers/acpi/apei/ghes.c | 65 ++++++++++++++++++++++++++++++++++++----
> 1 file changed, 60 insertions(+), 5 deletions(-)

...

> @@ -407,7 +447,22 @@ static void ghes_handle_memory_failure(struct acpi_hest_generic_data *gdata, int
>
> if (flags != -1)
> memory_failure_queue(pfn, flags);
> -#endif
> +
> + /*
> + * If the notification indicates that it was the interrupted
> + * instruction that caused the error, try to kick the
> + * memory_failure() queue before returning to user-space.
> + */
> + if (ghes_is_synchronous(ghes) && current->mm != &init_mm) {
> + callback = kzalloc(sizeof(*callback), GFP_ATOMIC);

Can we avoid that GFP_ATOMIC allocation and kfree() in
ghes_kick_memory_failure()?

I mean, that struct ghes_memory_failure_work is small enough and we
already do lockless allocation:

estatus_node = (void *)gen_pool_alloc(ghes_estatus_pool, node_len);

so I guess we could add that ghes_memory_failure_work struct to that
estatus_node, hand it into ghes_do_proc() and then free it.

No?

--
Regards/Gruss,
Boris.

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